1. Field of the Invention
The present invention relates generally to electrical engineering, and more particularly, to a method and apparatus for automatic gain control that can mitigate the effects of temperature and/or frequency.
2. Description of Related Art
Automatic gain control circuitry forms a portion of many radio transmitter circuits. Such circuitry can control the power level of a modulated signal transmitted by the radio transmitter circuit. Power level control is typically effected by controlling the gain of amplifier circuitry within the portion of the transmitter that amplifies the modulated signal.
Such gain control circuitry is often confronted with a task of maintaining a constant gain (i.e., "gain flatness") of a series chain of transmitter components over the entire operating frequency range. Gain flatness otherwise suffers due to gain or loss variations vs. frequency for most radio frequency (RF) and microwave transmitter components. For example, amplifiers, mixers, and filters often exhibit gain or loss variations across the entire operating frequency range. Moreover, gain or loss of typical transmitter components also varies as ambient temperature changes. The total variation at the output of the transmitter due to frequency variations and/or temperature fluctuations can become quite large and therefore out of a specified tolerance range. FIG. 1 illustrates typical gain vs. frequency and/or temperature of a series chain of transmitter components, which includes at least one conventional amplifier. Gain and output power typically fall off as frequency and/or temperature rise.
Generally, there are two ways to control these gain or loss variations. One such method is to specify each component of the transmitter to within a tight specification to achieve specified tolerances of gain flatness. However, this has proven costly and is often not readily achievable with current radio technology. Another method is to design a feedback loop that monitors the output power of the transmitter and constantly changes the gain so as to stay within a specified tolerance range. However, feedback loops increase the complexity of such transmitter circuitry and can also introduce stability problems that are commonly associated with such feedback loop circuitry.